During the manufacture of gun and cannon tubes it is necessary to press the tubes in order to straighten them. Pressing is when the tube is set on blocks with the curve side up. A ram pushes down on the deformed area until it starts bending and permanently deforms into a straight tube. By analogy it is similar to straightening a bent nail with a hammer.
Unpredictable factors such as residual stresses, varying material properties, complexity of bend, history of the heat treat, forging and dimensional variations are different with each tube. Therefore the operation requires a high amount of operator judgment. By knowing which of the four stages the metal is being subjected to, the operator can adjust his operation accordingly.
It has been determined that each stage has its own distinctly different type of acoustic emission. Under a no stress condition no emission comes from the metal. Only background, hydraulic, mechanical, and electrical noises can be detected. Hydraulic and mechanical noises have relatively long ringdown counts whereas electrical noises have very short ringdown counts. Elastic deformation is characterized by the intermittent acoustic emission due to matrix relocation, fracture of occluded particles and voids. Plastic deformation is characterized by continuous emission of medium length ringdown count emission due to gross amounts of plane slippage, relocation and small amounts of micro-cracking.
Cracking is characterized by very high amplitude, high energy spikes that are substantially louder than elastic and plastic deformation and background noise.